In the typical papermaking machine, an aqueous suspension of fibers is transformed into a paper web as it is processed through the different sections of the machine. One section of the papermaking machine is the dryer section wherein a wet paper web is passed about and held in intimate heat transfer relation with upper and lower arrays of heated cylinders in order to remove water from the paper web. The dryer section normally includes an upper and lower array of heated cylinders arranged and spaced in staggered, parallel rows which have a solid imperforate surface for contacting the paper web. Several dryer sections may be found in a papermaking machine in series and stages. The paper web is passed generally to and fro between the arrays of dryer cylinders in a generally serpentine manner to insure that both sides of the paper web contact the cylinders. As the paper web passes over the dryer cylinders, it is held in intimate heat transfer contact therewith by a belt, commonly referred to as a dryer felt or dryer fabric which has been made endless by techniques which are well known in the field of papermaker's felts and clothing.
In the past, dryer fabrics generally have been substantially impervious structures of either woven or needle construction. However, the impervious structures, commonly known as dryer felts, do not ventilate sufficiently and thus serve to confine heated vapors in certain "pockets" created in the dryer section which cause uneven drying and affect paper quality. Thus, the trend is toward open weave fabrics which have been found to have desirable characteristics and many non-woven structures, such as needle felts, plastic perforated and helical belts, and the like have also been found suitable due to their increased permeability. Typically, these plastic, non-woven fabrics have yielded permeabilities as high as a thousand cubic feet per minute.
The non-woven plastic spiral fabric is desirable because it has fewer if any "knuckles," as in the case of woven fabric, and thus provides increased surface area for contacting and holding the paper. This paper supporting surface is also smoother and reduces markings on the paper. The plastic material and belt construction hold up extremely well under the stresses encountered when traveling endlessly at high speeds, typically 3000 fpm, about the belt rollers in contacting the paper web.
The high permeability of non-woven plastic belting provides increased pocket ventilation and hence drying, but can also lead to increased fluttering of the paper web sheet against the dryer fabric through a phenomena known as air "pumping." This is due to frictional drag on the air to move with the fabric. As the fabric contacts the cylinders or belt rolls, this air is forced through the fabric with the air movement away from the roll or dryer at a converging nip and toward the roll or dryer at a diverging nip. The effect of these forces is the net inflow of air into a dryer pocket, resulting in an outflow of air at the front and back sides of the machine. This turnover of air or air "pumping" is appreciable with highly permeable fabrics travelling at high speeds which can cause disruptive sheet flutter. If a highly permeable dryer fabric is operated at high speeds with a paper web of low strength, the paper web may break, be damaged, or be marked by fluttering against the fabric.
The problem of controlling the permeability of the basic spiral fabric, particularly in the low range of fabric permeability, is one to which much consideration is needed. A low fabric permeability is required for many papermaking and drying applications. For example, fine paper grades have a relatively thin sheet and do not contain as much water. They do not require a dryer fabric of high permeability nor is such desired since paper flutter of the light sheet is likely.
In U.S. Pat. No. 4,381,612, issued to the assignee herein, it has been proposed to close the mesh of a plastic dryer fabric to reduce paper flutter by utilizing a plastic or low-melt monofilament nylon stuffer element wherein the low-melt nylon may be heat treated to expand the plastic material in the helix loops and provide a low permeability characteristic for the fabric. While this provides good results in the low range of fabric permeability, it is desired yet to provide fabric permeability characteristics even lower for the finer grades of paper, and, if possible, to do so without the additional step of heat treatment of the filler element.
U.S. Pat. No. 4,362,776 discloses an attempt to avoid the problems attendant to inserting a filler strand or strands through the loop openings of a helical belt to lower its permeability by winding the helix strips about the filler material prior to joining the helix strips in the belt. The wavy material also more completely fills the helix strip.
Accordingly, an important object of the present invention is to provide a construction for a plastic non-woven dryer fabric for a papermaking machine and method by which a desired permeability characteristic may be built into the fabric.
Another important object of the present invention is to provide a non-woven dryer fabric and method having a low fabric permeability characteristic.
Yet another important object of the present invention is to provide a filler element and method for closing the mesh of a helical dryer fabric which may be more easily inserted in the fabric.
Still another important object of the present invention is to provide a non-woven helical dryer fabric having an open mesh which is closed by a contoured stuffer element which occupies a substantial area of the loop of the helix owing to its contour.
Yet another important object of the present invention is the method and construction for a helical mesh wherein the open loop of the helix material is filled with a contoured filler element to provide a low range of fabric permeability without an additional step of heat treating the filler.